Transformer arrangement for coupling a communication signal to a three-phase power line

ABSTRACT

A communication signal is coupled simultaneously to all three phases of a three-power line through a three-phase open delta transformer with the communication signal being applied across the low voltage pair of windings.

BACKGROUND OF THE INVENTION

The present invention described and claimed in this continuation-in-partapplication which is based on parent application Ser. No. 934,449 nowU.S. Pat. No. 4,188,619 of like title filed on Aug. 17, 1978, pertainsgenerally to transmitting communication signals over three-phase powerlines and particularly to a transformer arrangement for coupling thecommunication signals to the power line.

Although the transmission of communication signals over three-phasepower lines, for example, to monitor and control the electric powersystem itself, has been successfully addressed heretofore, the need byelectric power companies for monitoring and controlling directlycustomer loads to combat increasingly intractable energy problems hascreated a need for new and different types of communication techniques.When the power line, itself, is used as a communications medium as analternative to telephone circuits or radio waves for transmitting datato and from utility customer sites, all of the customer loads served bya three-phase power distribution feeder are normally monitored andcontrolled from a central site via the distribution substation whichsupplies the feeder. Since these customer loads are individuallyconnected between one of the phase conductors and the neutral conductorin a three phase neutral-wye system or between two of the phases in athree wire delta system, the transceiver at the substation fortransmitting command signals to the customer loads and receiving monitorsignals therefrom must be capable of coupling the signal onto all threephases since the loads are always distributed among the three phases inorder to afford a balanced three phase load condition. This coupling maybe accomplised by the use of three transceivers, there being one forcoupling signals of a much higher frequency than the 60 hertz powerfrequency to each of the three-phase conductors or, alternatively, asingle transceiver which is sequentially connected to the three-phaseconductors so that all of the loads respectively connected thereto canbe connected to the transceiver for communication purposes. The formerapproach is not cost effective since it constitutes equipment redundancywhile the latter approach entails a switching mechanism which not onlyadds to the cost and diminishes the reliability of the equipment, butalso increases time required for broadcast commands and adverselyaffects the signal wave which is propagated down the power line. A thirdalternative is to couple the communication signal to all three phasessimultaneously such as described in U.S. Pat. No. 4,065,763, whichissued to Whyte, et al. The transformer arrangement depicted therein ofa single phase transformer with a multiple tap high voltage winding hasnotable disadvantages in that unbalanced phase conditions or tapsettings result in circulating communication signal currents in thetransformer which detracts from efficiency and the operation of thetransceiver which is already burdened with translating signals in adifficult environment.

In view of the foregoing, it is a primary object of the presentinvention to provide a new and improved transformer arrangement forcoupling a communication signal simultaneously onto all three phaseconductors of a three-phase power line.

It is a further object of the present invention to provide such a newand improved transformer arrangement which is relatively inexpensive andwhich optimizes the transceiver performance.

The foregoing objects, as well as others, and the means by which theyare achieved through the invention herein, may best be appreciated byreferring to the Detailed Description of the Invention which followshereinafter, together with the accompanying drawing.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the foregoing stated objects, the present inventionentails coupling a communication signal to all three-phase conductors ofa three-phase power line through a three-phase open delta transformerwith the communication signal being applied across the low voltage pairof windings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a first embodiment of the invention designed for use witha neutral-wye three-phase power line.

FIG. 2 depicts a second embodiment of the invention designed for usewith a delta three-phase power line.

DETAILED DESCRIPTION OF THE INVENTION

As symbolically shown in FIG. 1, a three-phase power distribution linedesignated generally by the reference numeral 10 consists of three phaseconductors A-C and a neutral conductor N. The distribution line 10,which most conventionally would assume a nominal voltage of 12 KV acrossany two phases thereof, is connected to the output terminals 12 of athree-phase step-down transformer 14 having low voltage windings 16conventionally arranged in a wye configuration and high voltage windings18 connected in a delta configuration. High voltage terminals 20 connectthe transformer 14 to the high voltage transmission line representedgenerally by reference numeral 22.

A transceiver 24 is located at the substation for generating and sendingcommand signals to be transmitted via the distribution line 10 toutility customer sites for controlling the loads and functions thereat,as well as for receiving signals from the customer sites in order tomonitor the loads. The transceiver 24, which may consist of a separatetransmitter and receiver or a single unit integrating both functions, iswell known in the art and need not be detailed herein. The transceiver24 block is intended to denote all of the equipment necessary forgenerating and modulating a carrier signal of suitable frequency, e.g.,3-10 kHz, for transmission to a customer site over the distribution line10, demodulating and detecting a carrier signal transmitted from thecustomer site over the power line 10 and further includes the attendantpower supply. The communication signal is coupled to or from thedistribution line 10 via low voltage terminals 25 through a transformerarrangement 26 consisting of a set of three like low voltage windings 28serially connected (in split delta fashion) across which thecommunication signal is applied or developed so that the same currentI_(L) flows through all three windings 28. A set of three like highvoltage windings 30 is connected in a wye configuration to the phasesA-C of power line 10 via high voltage terminals 31 with its neutralterminal connected to the neutral conductor N of power line 10 viaterminal 33. Each of the windings 30 is magnetically coupled to adifferent one of the low voltage windings 28 (by virtue of a single,three-phase transformer or three single-phase transformers) so that, asdenoted by the conventional dots of FIG. 1, the currents I_(H) inducedin high voltage windings 30 by the current I_(L) flowing throughwindings 28 all flow in the same direction with respect to the neutralconnection. It is to be noted that, since all three currents, I_(H),must be equal to one another since they are all proportional to the samecurrent I_(L) by the same turns ratio (the windings of each set beingalike), no circulating currents are developed in the transformer 26 asthe possible result of unbalanced phase conditions in the distributionline 10, thus avoiding any signal degradation which would otherwise becaused thereby. Moreover, the low voltage winding split deltaarrangement assures that the 60 hertz AC voltages induced in the lowvoltage windings 28 from the high voltage windings 30 are cancelled whenvectorially summed across terminals 25 so that no 60 hertz voltage isapplied to transceiver 24 except that due to residual unbalance. Thispermits the use of low voltage solid state equipment without the needfor artificially blocking the 60 hertz AC such as through a largecapacitor.

The utility customer loads are supplied from single phase feeders,generally designated as 32 from the distribution line 10, which areconnected to different phases thereof so as to impose on the line 10 anapproximately balanced load condition. After the distribution voltage isstepped down to customer utilization voltage, nominally 120/240 voltsthrough distribution transformers 34, the electricity is routed to thevarious customer sites through low voltage distribution circuits 36.Although the individual customer loads are not pictorially representedin FIG. 1, they would be connected to the low voltage distributioncircuits 36 at the same points as are transponders 38, there being anindividual transponder 38 provided for each customer site. Thetransponder 38, which is well known in the art and need not be describedin detail herein, responds to the command signals received fromtransceiver 24 by either effecting some load control function ortransmitting back to the transceiver 24 some load information which isbeing monitored. Accordingly, each transponder 38 is understood tocontain a transceiver, itself. Individual transponder control iseffectuated by encoding in the command signals the address of thetransponder 38 to which the command signal is directed, there being aunique address for each transponder 38. Since the command signals areapplied to all three phases A-C of the distribution line 10, it makes nodifference which phase the addressed transponder 38 is connected tosince it will always receive the signal over one of the three phases.Likewise, since the transceiver 24 receives response signals over allthree phases of the distribution line 10, it makes no difference whichtransponder 38 is transmitting since the signal will be received via theappropriate phase conductor.

In comparison measurements between single-phase and split delta couplingtechniques using standard distribution transformers at the substationand measuring between typical substation and residential locations, testdata shows the split delta configuration provides 3 dB to 6 dB advantagein transfer impedance in the 3 kHz to 10 kHz ranges. That is, the splitdelta configuration requires 3 dB to 6 dB less transmitted current toproduce the same received signal level in either direction. Thus, thesignal levels required for communication are materially reduced throughuse of the split delta coupling configuration.

Under conditions outlined above, the nominal 120-volt split deltatermination presents to the communication transmitter and receiver apower frequency fundamental (60 Hz) level of less than two volts rms ascompared to the full 120 volts rms with the single-phase termination.The level of the power frequency third harmonic (180 Hz) is dominant inthe split delta termination at typically less than five volts rms. From3 kHz to 10 kHz, power frequency harmonic levels of the twoconfigurations are comparable and less than one millivolt. Thus, withoutapparent operational penalty, the problem of blocking and protecting thetransmitter and receiver from the power frequency is notably mitigatedby the split delta coupling configuration.

An additional benefit of the aforedescribed arrangement is the reducedlikelihood that the communication signal applied to the distributionline 10 will interfere with the reception of the communication signalapplied to the distribution lines via the high voltage transmissionsystem including transformer 14. This is attributable to equal currentsI_(H) being applied to phase conductors 10A-10C, so that any equalcomponents thereof which pass through the low voltage windings 16 oftransformer 14 rather than out onto the line 10 merely induce acirculating current in the high voltage windings 18 which does not getcoupled onto transmission line 22 for transmission to other distributionpoints. Only in the event of an unbalanced impedance condition mightsome portion of the communication signal be so coupled.

A test at a distribution station showed that under like conditionsapplying the communication signal to all three phase conductors ratherthan a single phase thereof resulted in a 3 dB increase in signalattenuation, and consequently reduced cross-talk, as measured atsurrounding distribution stations.

It will be readily recognized that the transformer arrangement depictedin FIG. 1 is not suitable for a delta three-phase system. The problem oftransmitting communication signals over this type of system is obviatedby the transformer arrangement depicted in FIG. 2 wherein a three-phaseopen delta transformer arrangement 40 consists of two high voltagewindings 42 connected in open delta fashion to high voltage terminals 44while the two low voltage windings 46 are serially connected to lowvoltage terminals 48 across which the communication signal is applied ordeveloped. Since, in this arrangement, the 60 hertz AC voltages, inducedin the low voltage windings are not cancelled out, a capacitor 50interconnects the transceiver 24 with the transformer arrangement 40 toblock the application of high 60 hertz voltage to transceiver 24.

As the foregoing demonstrates, the transformer arrangement of thesubject invention allows communication signals to be coupled to and froma power distribution line using transceiving equipment which is ofconventional state of the art design, thereby affording costeffectiveness and simplified operation. Since, undoubtedly,modifications to the foregoing embodiments can be made by those skilledin the art without departing from the scope and spirit of the invention,the detailed description herein is intended to be merely exemplary andnot circumscriptive of the invention, which will now be claimedhereinbelow.

What is claimed is:
 1. A transformer arrangement for simultaneouslycoupling a signal to all three phase conductors of a three-phase powerline, comprising:a pair of high voltage windings serially connected,with their commonly connected leads being connected to one phase of thepower line and the uncommon lead of each winding being connected to adifferent one of the other two phases of the power line; a pair of lowvoltage windings serially connected across which the signal is applied,and means for magnetically coupling each winding of one pair with adifferent winding of the other pair so that the current flowing throughand between said high voltage windings is the same.
 2. Signalingapparatus for simultaneously coupling a signal to all three phaseconductors of a three-phase power line, comprising:an AC signal device;a pair of high voltage windings serially connected, with their commonlyconnected leads being connected to one phase of the power line and theuncommon lead of each winding being connected to a different one of theother two phases of the power line; a pair of low voltage windingsserially connected to said signaling device, and means for magneticallycoupling each winding of one pair with a different winding of the otherpair so that the current flowing through and between said high voltagewindings is the same.
 3. A signaling system, comprising:a three-phasepower line comprising three phase conductors; an AC signaling device; apair of high voltage windings serially connected, with their commonlyconnected leads being connected to one phase of the power line and theuncommon lead of each winding being connected to a different one of theother two phases of the power line; a pair of low voltage windingsserially connected to said signaling device, and means for magneticallycoupling each winding of one pair with a different winding of the otherpair so that the current flowing through and between said high voltagewindings is the same.